As much time as you have.
You will be able to see a lot in one afternoon, but it takes more time than that to hit every country in our experience. I agree that part of the fun of WS is not having a plan. Also, don't forget about the Agent P game. My kids love that. With that included, it could easily take my kids days and days to do EP. We have 3 days planned for our next trip at EP for 7 days worth of parks. And I'm sure we could easily spend more.
You will be able to see a lot in one afternoon, but it takes more time than that to hit every country in our experience. I agree that part of the fun of WS is not having a plan. Also, don't forget about the Agent P game. My kids love that. With that included, it could easily take my kids days and days to do EP. We have 3 days planned for our next trip at EP for 7 days worth of parks. And I'm sure we could easily spend more.